1. Field of the Invention
The present invention relates to an infrared sensor having a pyroelectric element and a field-effect transistor (FET).
2. Description of the Prior Art
An infrared sensor shown in FIG. 1 is conventionally known. In FIG. 1, reference numeral 1 denotes a pyroelectric element having pyroelectric effect; and 2, a field-effect transistor (FET) having drain, source and gate electrodes D, S and G. A diode Rg is used as a gate resistor, and a resistor Rs is used as a source resistor. Reference numeral 3 denotes a power supply terminal for supplying a power supply voltage +B to the drain electrode D; 4, an output terminal for producing a detection voltage V.sub.0 from the source electrode S; and 5, a grounding terminal.
According to the above configuration, a gate potential changes slightly in accordance with a change of infrared radiations detected by the pyroelectric element 1, thereby changing a drain current of the FET 2. Thus, when the drain current flows in the source resistor Rs, the detection voltage V.sub.0 can be obtained between the terminals 4 and 5.
In such an infrared sensor, a circuit shown in FIG. 1 is enclosed by a shielding case 6. The terminals 3, 4 and 5 are led out of the slielding case 6 with lead wires, respectively. These lead wires have an inductance with respect to high-frequency components. For this reason, if a strong high-frequency electric field, for example, due to radio communication in the UHF band is generated near the circuit, an electromotive force is easily induced in the lead wires. Meanwhile, a feedback capacitance Cr indicated by a dotted line in FIG. 1 is formed between the drain and gate electrodes D and G of the FET 2. In addition, a diode is formed by a p-n junction between the gate and source electrodes G and S of the FET 2. For this reason, a high-frequency current due to a voltage induced in the terminal 3 flows into the gate electrode G through the feedback capacitor Cr, and is detected by the above-mentioned diode. As a result, the detected low-frequency component is superposed on the detection voltage V.sub.0, or is output as the detection voltage V.sub.0 itself.
Furthermore, input and output capacitances Ci and Co are respectively formed between the gate electrode G of the FET 2 and ground and the drain electrode D and ground as indicated by dotted lines in FIG. 1. These capacitances Ci and Co are so small as to have high impedance with respect to high-frequency components. Therefore, this FET 2 may perform high-frequency amplification. When a high-frequency voltage is generated between the lead wires by an external high-frequency electric field, the potential thereof directly acts on the respective electrodes of the FET 2.
In this manner, the conventional infrared sensor is easily influenced by an external high-frequency electric field, resulting in a detection error.